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1960s GMC V12 Engine For Sale In Wyoming

GM V8 engines are world-famous for their inimitable blend of performance and reliability at an affordable price. However, one piece of the history of high-displacement GM engines that’s often forgotten is its V12. Specifically, the GMC “Twin Six,” a 702-cubic-inch (11.5L) V12 offered on GMC 7000 series trucks in the 1960s.

Now, hot-rodders have a rare chance to own an original example of this massive engine for their next project. Our friends at The Drive have noticed that a GMC Twin Six has appeared for sale on Facebook Marketplace in Casper, Wyoming, with an asking price of $4,500 (or best offer). It comes with the original transmission, which appears to be a Muncie SM420 4-speed manual. The other available transmission with this engine was an Allison heavy-duty automatic.

GMC Twin Six V12

The engine and transmission both look like they’ve seen heavy use. There’s quite a bit of surface rust on just about every visible component, but the seller claims it’s complete except for the starter. Since this engine shared dozens of components with GMC V6 engines of the same vintage, finding new parts might be easier than it sounds for such a rare engine.

As its name implies, the concept of the Twin Six was two GMC V6 engines mashed together. However, its design was a little more graceful than that. It had its own unique single block casting, four exhaust manifolds, two carburetors and intake manifolds, and two distributor caps. It originally only made 275 horsepower but a whopping 630 pound-feet of torque, making it a workhorse suitable for heavy-duty truck applications. According to ThunderV12, a Tennessee shop that specializes in this engine, approximately 5,000 examples of the Twin Six were built, and fewer than 200 remain.

GMC Twin Six V12

You may not have known this GM V12 engine existed before today. An LS swap is a dime a dozen and GM’s legendary V8s popular hot-rodding engines for good reasons. However, if you have an appetite for something different and a few extra cylinders, this Twin Six is waiting to breathe life into your next project.

George is an automotive journalist with soft spots for classic GM muscle cars, Corvettes, and Geo.

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Comments

  1. Museum piece at best.🙁

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  2. >> You may not have known this GM V12 engine exited before today.

    I love the proofreading in lots of modern articles where authors just assume no autocorrect errors exist.

    GM has left the building…

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  3. Would probably make a great engine for a tractor pull.

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  4. Wonder if it’s OBD II compatible?

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  5. I actually have seen this in person in a steel bodied hot rod, here where I live a guy who owned a machine shop built it and brings it to local car shows all the time. We all marvel at the car he built around this, it still sports the two carbs, runs great. But his story was that these particular engines were used in oil drilling rigs, and told me rigs that were beyond a certain depth like 325 feet deep or something. Making me think there were a lot of these out there in the oil fields. Anyway, the thing is a hoot to see start up, but compared to modern muscle is not that powerful, but it sure looks cool. Peace everyone.

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  6. one thing about those v6 engines do not turn them over 4400 rpm or they will blow

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  7. It looks like something that would finally fill the engine compartment of my ’72 Monte Carlo… especially now that I replaced the original fan & shroud with an electric fan!

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  8. I worked with guys who had driven these V12’s in tanker trucks for an oil company in the Midwest in the late 60’s or early 70’s.

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  9. The V6’s could get the Buick GN super charger?

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  10. A little searching shows this was a 11.5L engine (702 ci) produced 275 HP and 630 torque at 2,400 RPM
    Engine weighed 1485 lbs the crank weighed 190 lbs. first produced in 1960 and about 5000 total were made.

    In the same engine family, GMC offered a 635 ci V8 during the same period.

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  11. My dad had a ’63 GMC pickup with half of that engine (301 cubic inch V6) in it. It is a very interesting designed motor. While most V variety engines are of the 90 degree V design this engine is of the 60 degree design and the spark plugs are located on the intake manifold side of the heads. As stated by others this is a low RPM engine. It was hard to rev this engine over 3000 RPMs…..I know because I tried. I think it just ran out of carburetor at that point or maybe governed.

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  12. They were governed by oil pump that is where it was located

    Reply

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